In QGIS 3.12 I need to calculate the shortest distance from the boundary of one polygon to the boundary lines of a set of polygons as shown in the image below
I have followed the instructions according to this question Calculating shortest distance between multiple Polygon borders to a common Polygon border?.
It seemed to provide a good solution using the field calculator to generate a new feature in the attribute table (eg the attribute table of the blue polygons) using the following formula
distance($geometry, geom_from_wkt('Polygon ((........))'))
as described in the above link. However, when I randomly checked some of the distances using the measuring tool, while many of my tests were consistent, I found several that were incorrect e.g 196.11 km vs 216.9 km.
All the shapefiles and my project are in the same projection system.
I cannot figure out why there are the errors or if this is the best way to calculate these distances.
Ideally I would like tracklines generated to show the shortest distance, however this may be over complicated.